Niloy Chatterjee, who used the pen-name Niloy Neel, was murdered on Friday after the men broke into his flat in the capital's Goran neighbourhood, according to the Bangladesh Blogger and Activist Network, which was alerted to the attack by a witness

"They entered his room in the fifth floor and shoved his friend aside and then hacked him to death. He was a listed target of the Islamist militants," the network's head Imran H Sarker, told the AFP news agency.

Later on Friday, Bangladeshi newspaper The Daily Star, reported that Ansar al-Islam, a local chapter of al-Qaeda had claimed responsibility for the killing. The newspaper said the group had sent an email to media houses in Bangladesh, adding that the authenticity of the email issued by Mufti Abdullah Ashraf, who claimed himself to be the spokesman of Ansar-Al-Islam, could not be verified independently.

Chatterjee, 40, was a critic of religious extremism that led to bombings in mosques and the killing of numerous civilians, Sarker said.

Police confirmed Chatterjee, who lived with his family, had been murdered but had no details on his background.

In a recent Facebook post, Chatterjee said he was being followed by two strangers. He said he had approached the police but they had refused to register his complaint.

The United Nations special rapporteurs on freedom of expression, David Kaye, and on extrajudicial executions, Christof Heyns, condemned the blogger's murder.

"The violent killing of another critical voice in Bangladesh shows that serious threats to freedom of expression persist in the country," the pair said in a statement on Friday, calling for a prompt and thorough investigation.

"It is vital to ensure the identification of those responsible for this and the previous horrendous crimes, as well as those who may have masterminded the attacks."

There have been at least five bloggers attacked in Bangladesh since 2013, four of them since February of this year.

"There a is serious lack of inertia in part of Bangladesh authorities to investigate these thoroughly and find the actual culprits behind all these killings," Al Jazeera's Tanvir Chowdhury, reporting from Dhaka, said.

On May 12, Ananta Bijoy Das, another blogger, was killed in the Subid Bazar area of Sylhet city as he walked to in the morning.

In March, blogger Washiqur Rahman Babu was hacked to death in Dhaka by three men over alleged anti-Islamic writings, with two of the suspected attackers caught near the scene.

In February, Bangladeshi born American writer-blogger Avijit Roy was killed near the Dhaka University campus.

At least in two of the cases, the killers were apprehended, while in other cases only suspects were arrested. Authorities said their links to "radical Islamist groups" are "under investigation."

Sumit Galhotra, a spokesman at the New York-based Committee to Protect Journalists, told Al Jazeera that the latest incident "serve as a wake-up call" for the government, to do "everything necessary" to arrest and prosecute the attackers.

"Unless the government establishes accountability on these cases, these attacks will continue," he said, adding that many Bangladeshi bloggers are coming threat for their work.

In a recent Facebook post, Chatterjee wrote that he told police that he had been followed by unknown men, but they refused to register his complaint [Mahmud Hossain Opu/Al Jazeera]